The present invention generally relates to a coaxial high frequency cable and, in particular, relates to one such cable having a tubular metallic inner conductor and a tubular metallic outer conductor, the inner conductor and the outer conductor being fixed in their spatial position by a dielectric material.
In general, coaxial cables are well known in the art and are most often used for the transmission of high frequency signals. Typically, coaxial cables differ in the outer dimensions thereof and the dielectric material disposed between the inner and the outer conductors thereof. One factor in selecting the dielectric material is the power to be transmitted through the cable. However, a concentric arrangement and the energy transmission by electrical lines is common to all such cables. With the introduction of optical transmission elements, the connection of electrical transmission lines and optical transmission elements in parallel has been implemented. Such an arrangement has been implemented either to use the data transmitted via the optical transmission element for control purposes or to implement switching processes. Further, the optical transmission element can be used as a sensor to detect changes in the environment of the cable and to transfer the sensed signals to a central location for the purpose of controlling the operational status of the main cable, e.g. the operating temperature of an earth-bound high voltage cable.
Conventionally, for example, optical fibers are inserted in the interstices of the electrical power cables or, alternatively, one wire of the electrical braiding of the high voltage cable is replaced by an optical fiber. As a further alternative, particularly in the case of overhead transmission lines, an optical fiber is stranded with the single wires of the transmission line. This arrangement of light wave guide elements into the sandwich construction of electric cables provides the optical fiber with an excess length. As a result, stresses that occur during cable manufacturing and during cable installation do not result in fracture of the optical fiber.
However, typical high frequency coaxial cables differ from such electrical power lines even in their mechanical construction. Further, any steps or measures used in conjunction with the addition of an optical fiber must not be allowed to affect the electrical transmission characteristics of the coaxial high frequency cable.
Radiating high frequency cables, also called leading cables, are wave guides that typically include openings, for example, in the form of slits, disposed along the length of the outer conductor. The openings serve to radiate energy, introduced at one end of the cable, into the space surrounding the cable, where this energy is absorbed by appropriate aerial systems and the transmitted signals are evaluated. The output of the cable at the other end thus exhibits a decreased power intensity due to the natural attenuation of the cable and the continuous radiation therefrom. Frequently, such cables are mounted overhead in funnels, to provide the capability to passing traffic, such as trains or cars, to receive signals from the outside or, alternatively, to transmit signals from inside the tunnel to the outside. Many solutions are known to reduce the signal attenuation by varying the number, size, and arrangement of the slits.
However, in a typical tunnel, high frequency cables are not the only cables installed. For example, other data transmission lines are installed parallel thereto. Such data transmission lines may, for example, serve to control existing amplifier equipment. More recently, these data transmission lines are implemented by the use of optical fibers. However, the use of such optical fibers requires an additional cable installation. Further, each additional cable installation requires additional space, and in many instances, substantial added expense for mounting the cable where an attachment to the wall is not already available.
Consequently, the provision of a coaxial cable that overcomes the above-identified drawbacks of conventional cables is highly desirable.